thanks for the responses. locate doesn't find the files but as descendant_command highlights, that is because I have mounted a volume over the top of them. If I "umount" the volume then I can see the files in /mnt/extend and, yeah, root does look quite full.
So I umounted the volume on /mnt/extend and then deleted the files on what is now /mnt/extend. Bingo, a bit more free space.
And all the files are still present when I mount the volume again